1. Technical Field
The present disclosure relates to a high-strength Ni-base alloy.
2. Related Art
It is known that an Ni-base alloy having high tensile strength and creep strength at room temperature and high temperature can be obtained by a precipitation strengthening technique including aging an Ni-base alloy containing Al, Ti, and Nb. Such an Ni-base alloy is used in applications requiring high strength and corrosion resistance such as intergranular corrosion resistance around room temperature. In this precipitation strengthening, a fine precipitation strengthening phase, that is, a γ′ (gamma prime) phase and/or a γ″ (gamma double prime) phase, is precipitated in an austenite (γ) matrix phase. The γ′ and/or γ″ precipitation strengthening-type Ni-base super alloy, in which creep strength at high temperature is particularly emphasized, demands not only transgranular strengthening by the precipitation of a fine precipitation strengthening phase but also intergranular strengthening for inhibiting the occurrence of creep deformation. Therefore, carbides, intermetallic compounds, and the like are precipitated in order to strengthen grain boundaries. Also, elements such as P and S, which are trace impurities harmful to creep strength, are likely to be segregated at grain boundaries. Therefore, in order to inhibit these elements from being segregated alone, traces of carbides are precipitated at grain boundaries. That is, the trace impurities are removed from grain boundaries by, for example, a method of allowing the trace impurities to dissolve in this precipitate. For this reason, approximately 0.02% to 0.05% of C is often added purposely to allow traces of carbides containing Cr, Ti, Nb, and the like to be precipitated at grain boundaries and/or inside grains. Ti and Nb are also main constituent elements of the γ′ and/or γ″ phases that are precipitated during aging treatment and contribute to strengthening. Therefore, when primary carbides containing Ti and Nb exist in large quantities, the precipitation amounts of the γ′ and/or γ″ phases during aging precipitation decrease, possibly providing insufficient strength. For this reason, a little extra amounts of Ti and Nb are added in an alloy to be manufactured based on the premise that carbides are precipitated, in consideration of the amounts of Ti and Nb that are consumed into carbides.
On the other hand, the γ′ and/or γ″ precipitation strengthening-type Ni-base super alloy, to which Cr, Mo, and the like are added, has not only favorable strength but also favorable corrosion resistance around room temperature. Therefore, such an Ni-base super alloy is used in applications requiring high strength and corrosion resistance. As described herein, “around room temperature” refers to a temperature lower than a temperature at which creep occurs. For example, a temperature up to around 200° C. or 300° C. is also included in “around room temperature” as described herein. Corrosion resistance decreases due to the precipitation of carbides containing Cr at grain boundaries in a similar manner to stainless steel. Therefore, it is preferred that the content of C is low. A general γ′ and/or γ″ precipitation strengthening-type Ni-base super alloy used around room temperature often contains approximately 0.02 to 0.05% of C. The content of C is as low as the content of C for stainless steel. Accordingly, this Ni-base super alloy often exhibits favorable corrosion resistance.
For the γ′ and/or γ″ precipitation strengthening-type Ni-base super alloy used in specialized applications such as in nuclear power plants, a method of manufacturing a nuclear reactor internal member having improved stress corrosion cracking resistance is disclosed in, for example, JP-B-61-28746. In this method, a Laves phase (M2Nb) is disappeared, so that MC-type carbides (M is Ti, Nb, or the like) and a γ″ phase are precipitated in an austenite phase matrix. Furthermore, JP-B-4-42462 proposes an SCC-resistant Ni-base alloy member having improved stress corrosion cracking resistance (SCC resistance), and a method of heat treatment of the alloy member. This method leads to precipitation at grain boundaries of M23C6-type carbides that are coherent to austenite crystal grains, so that the grain boundaries have a zigzag shape.